A low-cost hydrogen storage technology that provides a high storage capacity and fast kinetics is a critical factor in the development of a hydrogen economy for transportation. The solid-state storage is now considered as the safest and most effective way of routinely handling hydrogen(1,2), and the attention is focused on metal hydrides(3), complex hydrides(4-7), nano-tubes and fibers(8-16), micro-porous metal-organic materials(17), and lithium nitride(18-21). A hydrogen storage technology which can economically carry enough hydrogen on-board of a vehicle to enable a 300-mile vehicle range is critical to make the hydrogen-powered automobiles competitive with the traditional vehicles. Furthermore, the DOE mid-term target for on-board hydrogen storage material is 6 wt % reversible hydrogen capacity with fast kinetics. At the present time, no existing hydrogen storage material meets this target.
As early as 1910, Dafert and Miklauz reported that Li3N can absorb 10.4 wt % hydrogen to form Li3NH4 (22) (Li3N+2H2=Li3NH4) and the Li3NH4 can decompose to release hydrogen. Furthermore, Ruff and Goeres reported that Li3NH4 is a mixture of LiNH2 and 2LiH (23). Therefore, Li3N can be a useful storage material. However, it did not attract attention for about a century probably because of the suspicion that it can generate NH3, which, indeed, is a thermodynamically favorable process at temperatures below 400° C. (18a). However, recent experiments showed that no NH3 could be detected during the hydrogenation of Li3N and the dehydrogenation of hydrogenated Li3N (18, 21). Furthermore, recent experiments demonstrated that an ultra-fast reaction between NH3 and LiH enables LiH to capture the entire NH3 generated during hydrogenation and dehydrogenation (18a,19). Thus, Li3N has recently started to attract attention as a material for hydrogen storage(18-21). However, a critical issue is that its reversible hydrogen capacity is less than 5.5 wt %. This occurs because LiNH2 and 2LiH, which are the products of Li3N hydrogenation, dehydrogenate in two steps: LiH+LiNH2=Li2NH+H2 and LiH+Li2NH=Li3N+H2. The first step, which provides about 5.5 wt % hydrogen capacity, takes place easily even at temperatures below 200° C., whereas the second step requires high temperatures (>400° C.). Furthermore, it has been found that Li3N undergoes the binding of hydrogen at such a rate that the heat released in the binding reaction causes hot spots in the solid, resulting in sintering of the solid and a corresponding decrease in its hydrogen capacity, reversibility, and thus its usefulness as a storage medium. A stable hydrogen storage medium which has a high storage capacity and a high reversibility would be a significant advance in the storage of hydrogen, particularly for use in portable hydrogen fuel cells.